Hydraulic valve clearance compensation elements serve to compensate for the clearance which, due to wear or thermal expansion, forms between the transmission elements transmitting the cam lift to the gas exchange valves of the internal combustion engine. The intention is to achieve a quiet and wear-resistant valve train and the greatest possible conformity between the cam lobe and the valve lift.
Hydraulic valve clearance compensation elements have a control valve in the form of a non-return valve, which comprises a control valve ball and a control valve spring acting thereon. In the standard type of control valve the control valve spring acts on the control valve ball in the closing direction. This largely closes the control valve and there is no idle lift of the valve clearance compensation element. There is even a risk of pumping up the compensation element and of a negative valve clearance.
These disadvantages are avoided by control valves, the control valve spring of which acts upon the control valve ball in the opening direction. Because of the reversed arrangement of the control valve spring, hydraulic valve clearance compensation elements comprising such a control valve are referred to as hydraulic reverse spring valve compensation elements (RSHVA). These have a positive influence on the thermodynamics, the pollutant emissions and the mechanical stressing of the internal combustion engine and are therefore being increasingly used.
In the standard design type, the control valve is largely closed in the base circle area of the cam owing to the force of the control valve spring. In an RSHVA the control valve in this area is kept open by the force of the control valve spring. Since the RSHVA can only be closed by the hydrodynamic and hydrostatic forces due to the flow of lubricating oil commencing at the beginning of the cam lobe and flowing from the high-pressure chamber to the low-pressure chamber, the RSHVA always has an idle lift before the valve lift commences. The extent of the idle lift at any engine speed depends on the length of the RSHVA closing time and this in turn depends on the viscosity of the lubricating oil.
To close the control valve of an RSHVA, a so-called critical lubricating oil velocity is required. This varies as a function of the lubricating oil viscosity and hence of the lubricating oil temperature. At high lubricating oil viscosity, that is to say at low lubricating oil temperatures, the critical lubricating oil velocity is lower and is therefore attained more rapidly than at low lubricating oil viscosity, that is to say high lubricating oil temperatures. In cold starting this leads to a shorter closing time of the control valve and hence to a smaller idle lift than in the engine at operating temperature. A small idle lift means a large valve overlap, however. This results in a large internal exhaust gas recirculation, which causes an uneven, low idling. Although this can be improved by increasing the idling speed, this is achieved at the expense of the pollutant emissions and the fuel consumption.
The generic EP 1 298 287 A2 discloses an RSHVA for the valve train of an internal combustion engine, which is characterized by the following features:                a housing, which has a blind bore, in which a piston is guided with a tight, sealing clearance;        the piston comprises a lower piston part with a lower piston head, which together with the blind bore defines a high-pressure chamber, whilst a low-pressure chamber is situated above the lower piston head;        the pressure chambers are connected by means of a central axial bore in the lower piston head, which is controlled by a control valve arranged on the underside of the lower piston head;        the control valve comprises a control valve ball, upon which a control valve spring acts in the opening direction and the lift of which is limited by a lift-limiting stop of a valve ball cap.        
This published patent application focuses on the optimum design for the control valve spring of the RSHVA. The design is selected so that the control valve is open in order to facilitate fitting of the RSHVA in said valve and so that it allows an exchange of fluid between the high-pressure chamber and the low-pressure chamber, but in the event of a pressure rise in the high-pressure chamber will permit a rapid closing of the control valve against the spring force of the control valve spring.
The influence of the lubricating oil temperature on the closing time of the control valve and hence on the idle lift of the RSHVA does not form the subject matter of this published patent application.
U.S. Pat. No. 4,054,109 describes an RSHVA, the features of which largely correspond to those of EP 1 298 287 A2. This patent specification focuses on a valve lift that increases with the engine speed. This increases inversely with a reduction in the idle lift of the RSHVA. The start of the valve lift depends on the attainment of a specific rate of lift of the valve-actuating cam, which serves to close the control valve. Since the required rate of lift is attained ever earlier as the engine speed increases, the valve lift becomes correspondingly greater as the engine speed increases, whereas the idle lift of the RSHVA correspondingly diminishes.
This specification also fails to mention any influence of the lubricating oil temperature on the idle lift, closing time and valve overlap.
The Japanese published patent application 61 185 607 A discloses an RSHVA which in construction and function approximates to U.S. Pat. No. 4,054,109. In contrast to the latter, the reverse spring of the Japanese application takes the form of a disk spring rather than a helical coil spring. In this application, too, the idle lift of the RSHVA diminishes with increasing engine speed, and the lift of the gas exchange valves and hence the engine power output diminish correspondingly. The smaller valve lift at low engine speeds is intended to reduce the fuel consumption.
In this specification, too, no reference of any kind is made to the lubricating oil temperature exerting an influence on closing time of the control valve and the idle lift of the RSHVA.